The present invention generally relates to a contact type image sensor which is applicable to an optical reader provided in a copying machine, a facsimile machine or the like.
An image sensor is used in an optical reader for optically reading characters and graphics on a document. Conventionally, an image sensor includes a lens system, which prevents miniaturization of the optical reader. Recently, a full-size image sensor capable of reading a document of A4 or B4 size without magnifying an optical image of the document is available.
Generally, a full-size image sensor includes a sensor part (light receiving part) and a drive circuit formed by thin-film transistors (TFTs). The sensor part and the drive circuit are integrally formed on a base. An incident light emitted from a light source enters the image sensor through a window formed therein. Then the incident light leaves the image sensor and is irradiated onto a document to be scanned and then reflected thereby. The reflected light enters the image sensor again, and is irradiated onto the light receiving part thereof. The image sensor except the window is covered by a light interrupting member used for interrupting the incident light. Particularly, the light receiving part of the image sensor must be completely covered by the light interrupting member so that no incident light is irradiated directly onto the light receiving part.
However, currently available full-size image sensors have the following disadvantages. FIG.1(A) is a waveform diagram of an output signal derived from a conventional full-size image sensor. A letter N indicates noise contained in the sensor output. FIG.1(B) is a waveform diagram of an ideal sensor output S having no noise. It can be seen from FIG.1(A) and FIG.1(B) that the signal-to-noise ratio of the sensor output from the conventional full-size image sensor is poor. Thus, it is difficult to read the document with high precision.